System and method for AC capacitance sensing of pavement density

ABSTRACT

The present invention discloses a system and method of determining the density of pavement material. The invention includes positioning in the AC capacitive sensor adjacent to a sample of pavement material, generating an electric field between the sensor and the pavement material, measuring the capacitance detected by the sensor, and correlating the capacitance to density of the pavement material. The invention further discloses determining a location and associating the location with a pavement density.

BACKGROUND OF THE INVENTION

[0001] This invention relates to non-destructive sensing of pavementdensity. More specifically, the invention relates to use of ACcapacitive sensing to determine pavement density.

[0002] In paving, asphalt pavement is often used. In the asphalt pavingprocess, various grades of aggregate are used. The aggregate is mixedwith asphalt cement (tar). There are also air voids within the mix. Ahot asphalt mix as laid has more air voids than is desirable. A paverlays down the asphalt mix and levels the asphalt mix with a series ofaugers and scrapers. The material as laid is not dense enough due to thenumber of air voids in the asphalt mix. Therefore, a roller makes anumber of passes over the layer of asphalt material (mat), driving backand forth, or otherwise creating sufficient compaction to form asphaltof the strength needed for the road surface.

[0003] There are a number of problems associated with this process ofpaving. These problems relate to determining when there is sufficientcompaction of the asphalt. If the asphalt is not sufficiently compacted,the quality of the pavement suffers. The amount of compaction isdependent upon a number of factors including the asphalt cement content,the aggregate type, the aggregate size, the aggregate shape, theaggregate texture, the distribution gradation, filler content, the mixtemperature, the speed of the roller, the number of passes of theroller, irregularities in the roller performance, and other factors.

[0004] It is important for the asphalt mat to be of proper density. Ifthere is not proper density, there will be long term deterioration whenthe asphalt begins to crack or there may be other problems related tostability and durability.

[0005] The density of the asphalt mat is increased through repeatedcompression with the roller. Preferably, the asphalt is uniformly densethroughout. There are numerous problems associated with determining thedensity of the asphalt so that one can be assured of the resultingstability of the asphalt mat.

[0006] One method of determining density is through use of a nucleargage. In this process, a number of different measurements are taken fromdifferent locations on the asphalt mat. Nuclear density measurementsystems suffer from any of the following; high cost, safety issues ofradioactive materials, need to drive a probe or spike into the mat;thereby leaving a hole in the mat, inability to measure density on acontinuous basis, unfeasibility of installing a plurality of suchmachines on a piece of road construction equipment.

[0007] Other approaches have also been used to measure density. One suchapproach is a Capacitance Energy Dissipation (CED) method. The CEDapproach involves using a capacitor charged to a voltage to energize aplate in contact with the asphalt. Direct current then flows from thecapacitor into the ground as the capacitor discharges. The time requiredto discharge the capacitor is measured and this time is compared to thedischarge rate of a reference capacitor with a known discharge rate.There are also a number of problems associated with this method. Oneproblem is that this method is dependent upon temperature. Anotherproblem is that the capacitor plate must come in contact with thepavement, so that the contact plate would need to be periodicallycleaned. Another problem is that there is a time associated with thedischarge rate that may impede the ability to continuously measurepavement density.

[0008] Therefore there are a number of problems associated withmeasuring pavement density. Thus, it is a primary object of the presentinvention to provide an improved system and method for measuringpavement density.

[0009] Another object of the present invention is to provide a systemand method for measuring pavement density that does not require a sensorto come into contact with the pavement.

[0010] Another object of the present invention is to provide a systemand method for measuring pavement density that provides fornon-destructive evaluation.

[0011] Yet another object of the present invention is to provide asystem and method for measuring pavement density that is more sensitiveto density of the top portion of the mat.

[0012] A further object of the present invention is to provide a systemand method for measuring pavement density that reduces cost andcomplexity.

[0013] Another object of the present invention is to provide a systemand method for measuring relative pavement density.

[0014] These and other objects of the present invention will becomeapparent from the following description.

BRIEF SUMMARY OF THE INVENTION

[0015] The invention is a system and method for capacitive sensing ofpavement density. One or more AC capacitive sensors are positionedadjacent to an asphalt mat. The measurements from the sensors arecorrelated to particular asphalt densities. Based on these densities, itis known whether further compaction is required.

[0016] One aspect of the invention provides for multiple sensors to beused simultaneously. This permits density measurements to be taken atmultiple locations. The density of different locations may also becompared to determine if the density is uniform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a diagram of the pavement density sensing system of thepresent invention.

[0018]FIG. 2 is a diagram of a capacitive sensing system with multiplesensors.

[0019]FIG. 3 is a diagram of a display of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020]FIG. 1 is a diagram showing an overview of the AC capacitivesensing system. In FIG. 1, there is an AC capacitive sensor 10. The ACcapacitive sensor 10 may be a BCAW-030-NB-1-Y-3 or other capacitiveproximity sensor. This sensor has an analog output, the magnitude of theanalog output is a function of the sensed capacitance. The presentinvention contemplates that other capacitive proximity sensors providingoutput signals, whether current or voltage, may be used. The presentinvention also contemplates that the output may be digital provided thatsufficient resolution is available. The capacitive proximity sensor usedis of a dielectric type as opposed to the conductive type. Thecapacitive sensor is positioned above the asphalt mat 12. A distance 14separates sensor 10 and asphalt mat 12. The sensor 10 generates anelectric field. The presence of the asphalt mat 12 in a close proximityto the sensor 10 increases the capacitance measured by sensor 10. Thecapacitance sensed is related to the distance 14 as well as thedielectric constant of the material being sensed.

[0021] The distance between the sensor 10 and the asphalt mat 12 shouldbe a fixed or known distance. Tests have indicated that a distance ofapproximately one centimeter is effective. However the present inventioncontemplates that a range of distances may work, for example just abovecontact such as approximately one-half millimeters to a distance of tenor more centimeters. The material being sensed is the asphalt pavement12 and potentially the material under the asphalt pavement 12, such asthe pavement base 16 and the soil 18. The AC capacitive sensor 10 ismore sensitive to the top most portion of the asphalt mat. For strengthand stability purposes, it is most important that the top most portionbe of proper density.

[0022] Tests were conducted using pavement having differences inpavement density. In the tests, a multimeter was used to measure theoutput current of a sensor, the current produced related to thecapacitance measured. When the capacitances of each asphalt pavementsample was measured at an identical distance 14, the denser asphaltsample had a higher capacitance than the less dense asphalt sample. Inother words, the denser pavement sample had a higher dielectric constantthan the less dense asphalt sample.

[0023] The present invention contemplates that the sensor 10 is capableof being adjusted for sensitivity. Adjustment involves increasing ordecreasing the distance between the sensor and the asphalt mat. Inaddition, adjustment may also involve making adjustments to the sensoritself including to the signal conditioning of the sensor such as isassociated with a potentiometer or other adjustment which is a part ofthe sensor. The sensor 10 is electrically connected to a controller 22.The controller 22 is a micro processor, a micro controller, a computer,a digital signal processor, a circuit, a processor, an integratedcircuit, a portion of an integrated circuit, or other control. Thecontroller 22 receives the sensed capacitance measurement. The presentinvention contemplates that depending on the particular controlsselected and the particular capacitive sensors selected, signalconditioning and/or analog to digital conversion may need to beperformed. Once received, the controller can perform numerous operationson the capacitance measurements. For example, the controller 22 canstore the measurements to a memory or data storage unit 24. The datastorage unit 24 can be any of a number of types of storage includingEPROM, EEPROM, flash memory, magnetic memory, optical memory, and othermemory or storage devices. The data stored may later be compared orcorrelated with measurements calculated or measured in other ways, suchas through core samples. This allows one to calibrate the sensor 10accordingly if required or made desirable by different distances 14,different moisture content, different thicknesses of asphalt pavement,different mixtures of asphalt, or other reasons.

[0024] The controller 22 is optionally attached to network interface 20.The network interface 20 allows the controller 22 to communicate over anetwork. For example, the network can be a Controller Area Network(CAN). The present invention however contemplates that any number ofnetworks may be used in order to communicate information pertaining topavement capacitance, density, or other measurements or diagnosticinformation from the controller to another device and/or location. Inaddition, the present invention contemplates that a locationdetermination device such as a GPS receiver 21 is operatively connectedto the controller 22. In this manner, the controller 22 can receivelocation information and associate that information with measurementsand/or calculated information.

[0025] In addition, the controller 22 is operatively connected to thedisplay 26. The controller 22 may correlate the capacitance measured bysensor 10 to a pavement density. The capacitance over the pavementdensity is displayed on the display 26. The present inventioncontemplates that the density calculated, stored, and/or displayed maybe a relative density.

[0026]FIG. 2 shows the system of the present invention with multiple ACcapacitance sensors 10. The AC capacitance sensors 10 are mounted on asupport structure such as a bar 28. The controller 22 is connected toall of the AC capacitive sensors. This permits the controller 22 toreceive sensor measurements for each sensor and each portion of asphaltpavement associated with each sensor. The controller 22 calculates arelative pavement density. For example, the controller 22 compares thecapacitances of each of the sensors to determine the pavement portionwith the lowest capacitance and thus the lowest density. Similarly, thecontroller 22 determines the asphalt pavement portion with the highestcapacitance and therefore the highest density. If the multiplecapacitance readings are relatively close, then the controllerdetermines that the density is uniform. By comparing the measureddensity of each strip of pavement with the desired density, thecontroller determines if additional compaction of the asphalt pavementis required for a particular portion of the asphalt pavement.

[0027] The controller 22 can also correlate the capacitance directly toa density. Then the controller calculates the densities associated witheach portion of the asphalt pavement and displays these densities on thedisplay 26. The controller 22 then compares these calculated densitiesto a threshold density. The threshold density can be a minimal densityor a maximum density and can be a range of densities. For example, thethreshold density may be any density within a set defined by as thosedensities between a minimum density and a maximum density. The thresholddensity is a known density or relative density. For example, thethreshold may be defined as a density determined experimentally. Thereis a capacitance associated with the density which is the thresholdcapacitance. The precise density need not be numerically calculated,instead the capacitance of a sufficiently dense pavement sample is usedto set the threshold density. Thus, in this manner, the capacitance iscorrelated to a density. Alternatively, densities are calculated. Thepresent invention contemplates that measured capacitances are correlatedto relative densities or to actual densities that are mathematicallycalculated for a particular capacitance or are estimated, looked up inan experimentally defined table or otherwise defined. If the calculateddensities are less than the threshold density then the controller causesthe display 26 to display a message that further compaction is required.

[0028] When multiple sensors are used, each sensor is associated with aparticular portion or strip of asphalt pavement. Preferably, thecapacitive sensors simultaneously sense the capacitance associated withthe respective strips of asphalt pavement.

[0029] The display includes a bar graph display with a bar associatedwith each strip of asphalt pavement. Based upon the bar graph display,an operator determines which strips of pavement require additionalcompaction.

[0030] The present invention contemplates that the support structure 28is located on, or is a part of a roller. Thus, as the pavement is beingcompacted by the roller, the density of the pavement is monitored todetermine whether additional passes need to be made over the asphaltmaterial to compact it or to determine whether additional passes need tobe made only over certain portions of the asphalt pavement material. Inaddition, this density data is stored in data storage 24 to providedocumentation that the pavement is of sufficient density to comply withconstruction standards.

[0031] The present invention contemplates that one or more capacitivepavement density sensors can be installed on a roller or other machine.A location receiver, such as a Global Positioning System (GPS), GLOSNASSor other navigational receiver or similar device can be installed on themachine to determine machine position. The machine position is thenrecorded while measuring pavement density therefore creating a record offinal pavement density associated with particular locations. The roadsurface is then broken into a grid with each element or cell of the gridhaving a dimension of 20 cm. The present invention is not limited to aparticular type of navigational receiver or to a particular size ofgrid. The present invention contemplates numerous variations such as maybe convenient or appropriate given a particular set of circumstances.The record of the pavement density and location is used to document if astrip of pavement fell between roller passes and was not rolled or wasnot sufficiently rolled. This system allows and enables documentation ofthe quality of the pavement of every grid location.

[0032]FIG. 3 shows a diagrammatic representation of a display 26 of thepresent invention. Display 26 includes multiple bars 32 associated withcapacitive sensors. For example, FIG. 2 shows AC capacitive sensor 10Athat measures capacitance of asphalt pavement strip 30A. FIG. 3 showsbar 32A which is a relative density of asphalt pavement strip 30A.Similarly, in FIG. 3 densities associated with other strips of asphaltpavement are shown. In addition, a line 34 is set according to arequired pavement density, a capacitance associated with a pavementdensity or other threshold. Thus, a roller operator is apprised as towhich portion of the asphalt pavement requires additional compaction. Ifbar 32A falls below required pavement density line 34, the asphaltpavement strip associated with bar 32A requires additional compaction.

[0033] Thus, a method and system of pavement density sensing has beendisclosed. The present invention contemplates numerous variations in theparticular AC capacity sensors being used, the distance between capacitysensors and asphalt pavement, the controller used, the display used, thedata storage used, and other variations.

What is claimed is:
 1. A method of determining the density of pavementmaterial comprising: positioning an AC capacitive sensor adjacent to asample of pavement material; generating an electric field between thesensor and the pavement material; measuring the capacitance detected bythe sensor; and correlating the capacitance to the density of thepavement material.
 2. The method of claim 1 further comprising adjustingthe sensitivity of the capacitive sensor.
 3. The method of claim 1further comprising adjusting the measured capacitance to compensate formoisture content of the pavement material.
 4. The method of claim 1further comprising recording the measured capacitance.
 5. The method ofclaim 1 further comprising adjusting the sensitivity of the capacitivesensor.
 6. The method of claim 1 wherein the step of correlating iscomparing the measured changes associated with the sample pavementmaterial with measured changes associated with a second sample ofpavement material, the density of the sample relative to the density ofthe second sample being known.
 7. The method of claim 1 furthercomprising determining a location and associating the location with apavement density.
 8. The method of claim 7 further comprising storingthe location and the associated pavement density.
 9. A system formeasuring pavement density comprising: at least one AC capacitive sensorcapable of outputting a signal; a controller operatively connected tothe AC capacitive sensor; and the controller receiving the signal fromat least one AC capacitive sensor, correlating the signal to a pavementdensity.
 10. The system of claim 9 further comprising a display, thedisplay operatively connected to the controller, the controller sendinga signal to the display, the signal related to pavement density.
 11. Thesystem of claim 9 further comprising a data storage component, the datastorage component operatively connected to the controller, thecontroller sending data to the data storage component.
 12. The system ofclaim 11 wherein the data is AC capacitive sensor measurements.
 13. Thesystem of claim 10 wherein the data is pavement density.
 14. The systemof claim 9 wherein the pavement density is a relative density.
 15. Thesystem of claim 9 further comprising a location determination device,the location determination device operatively connected to thecontroller.
 16. The system of claim 15 wherein the locationdetermination device is a GPS receiver.
 17. The system of claim 15further comprising a memory operatively connected to the controlleradapted to store pavement density data and location data.
 18. The systemof claim 15 further comprising a display operatively connected to thecontroller adapted to display pavement density data and location data.19. The system of claim 9 further comprising a network interfaceoperatively connected to the controller.
 20. The system of claim 19wherein the network interface is a controller area network interface.